The MRI apparatus is an imaging diagnostic apparatus which magnetically excites nuclear spins of an object set in a static magnetic field with RF (radio frequency) signals having the Larmor frequency and reconstructs an image based on MR (magnetic resonance) signals generated due to the excitation.
In order to obtain images with a high SNR (signal to noise ratio) by an MRI apparatus, it is important to set the optimal reception gain of MR signals. If a reception gain of MR signals is not set appropriately, MR signals may be saturated in an imaging scan (a scan for acquiring diagnostic image data). In addition, large noises due to quantization or the like lead to deterioration in SNR. In this case, it may become impossible to generate images effective for a diagnosis.
The optimum reception gain of MR signals allows receiving MR signals using a large part of a dynamic range of an A/D (analog to digital) converter (ADC). However, intensities of MR signals change depending on factors, such as imaging conditions. Hence, the reception gain of MR signals is conventionally set for every imaging.
Specifically, a pre-scan (a scan preceding to an imaging scan) limited to an imaging area where MR signals are acquired with high intensities is performed under same conditions as those of an imaging scan. In the pre-scan, MR signals are acquired with a low reception gain which has been set as an initial value. Then, a reception gain RGN for an imaging scan can be determined by formula (1) based on the maximum value of the MR signals acquired in the pre-scan.RGN=20 log10(DR/preMax)+preRGN  (1)
In formula (1), DR represents a dynamic range of an A/D converter, preMax represents the maximum value of MR signals acquired in a pre-scan, and preRGN represents an initial value of the reception gain.
In a pre-scan for determining a reception gain, MR signals whose echo number is smaller than that in an imaging scan are acquired. However, a pre-scan contributes to an increase in examination time since the pre-scan is performed for every imaging. Therefore, improvement in throughput of examinations is desired.
Accordingly, an object of the present invention is to provide a magnetic resonance imaging apparatus and a magnetic resonance imaging method which can determine a reception gain of MR signals in a shorter time.